A heater tube, for such a pressurizer of a primary cooling system, normally comprises an outer metal casing that has an elongated cylindrical shape (for example 22 mm diameter by 2 m long approximately) called a “sheath”, and a heater mounted inside the sheath.
Such tubes are mounted on a lower portion of the pressurizer, as explained in detail in document FR-2 895 206, and are submerged in the water of the primary cooling system that contains the pressurizer. They are used to raise the operating pressure of the primary cooling system. It will thus be understood that the tubes are under load when in use and undergo, in particular, thermal stress which, when combined with work-hardening stress as described below, potentially causes stress corrosion.
Incidents have shown that leaks could occur on the heater tubes of the state of the art. In particular, the sheath of a tube can crack, such that the inside of the tube is open to the water present in the pressurizer. There follows a possible deterioration of the heater of the tube, loss of operation of the tube, or even the leaking of the pressurized water to the outside of the pressurizer, through the inner space of the tube.
As a result, a solution is sought for limiting the risks of cracking of the sheath, due in particular to the stress corrosion that the sheath undergoes.
A solution was proposed in the above-mentioned document FR-2 895 206 that aimed to deposit a protective nickel coating onto the external surface of the sheath, using electrolysis. However, the solution of adding material leads to an alteration in the geometry of the tube, in particular by increasing the diameter thereof. Moreover, the solution is not completely secure, as the risk of removal of the nickel layer under the effect of impacts or friction cannot be ruled out. Given the increase in diameter, this solution cannot be used with heater tubes that are already manufactured, as they may no longer match the dimensions of the supports. Furthermore, it is lengthy to implement.
The present invention aims to improve the situation.